AG Perarivalan, who was convicted of supplying batteries for the belt-bomb that killed the former Prime Minister in 1991, has said that the CBI dropped the part in his confession where he had said he had "absolutely no idea" what the batteries were for.
The court today asked the centre and the CBI to get back to it within two weeks on whether Perarivalan should be freed.
Perarivalan's lawyer Gopal Sankaranarayanan told the court that while his client had been jailed for supplying two nine-volt batteries to make the bomb, the CBI had dropped a part of his confession that indicated he was not aware of the assassination plot.
The lawyer quoted an affidavit filed by V Thiagarajan - a senior CBI officer at the time - who said that he hadn't questioned Perarivalan properly. He also told the top court that the person who made the bomb is in Sri Lanka, and hadn't been questioned yet by investigators.
Perarivalan was 19 when he was arrested weeks after the assassination.
He spent 14 years in solitary confinement after being sentenced to death. His death sentence was changed to a life term by the Supreme Court in 2014.
A multi-agency probe was set up in 1998 on the recommendation of the Jain Commission which investigated the conspiracy.
Rajiv Gandhi was assassinated by a woman operative of the separatist Lankan Tamil outfit LTTE, who greeted him at a rally in Sriperumbudur town with a bomb strapped to her chest.
Perarivalan's request will be taken up on December 6.